Silica Microcavity Research Articles

Van der Waals Decoration of Ultra-High-Q Silica Microcavities for χ(2)-χ(3) Hybrid Nonlinear Photonics.

Optical nonlinear processes are indispensable in a wide range of applications, including ultrafast lasers, microscopy, and quantum information technologies. Among the diverse nonlinear processes, second-order effects usually overwhelm the higher-order ones, except in centrosymmetric systems, where the second-order susceptibility vanishes to allow the use of the third-order nonlinearity. Here we demonstrate a hybrid photonic platform whereby the balance between second- and third-order susceptibilities can be tuned flexibly. By decorating ultra-high-Q silica microcavities with atomically thin tungsten diselenide, we observe cavity-enhanced second-harmonic generation and sum-frequency generation with continuous-wave excitation at a power level of only a few hundred microwatts. We show that the coexistence of second- and third-order nonlinearities in a single device can be achieved by carefully choosing the size and location of the two-dimensional material. Our approach can be generalized to other types of cavities, unlocking the potential of hybrid systems with controlled nonlinear susceptibilities for novel applications.

Hydrophobic Silica Microcavities with Sustainable Nonlinear Photonic Performance.

Ultrahigh quality factor (Q) microcavities have been emerging as appealing compact photonic platforms for various applications. The Q factor plays a critical role in determining the nonlinear optical performance of a microcavity. However, a silica microcavity suffers from severe degradation of its Q value over time during storage or use in air due to the accumulating surface absorption loss, which would deteriorate their nonlinear photonic performance. Here, we report a new type of ultrahigh Q silica microcavity that effectively prevents Q degradation over time. The Q values of the devices remain unchanged over time under storage in air. Optical frequency combs are generated with sustainable ultralow threshold performance over the course of time from the devices in open air. This approach would greatly facilitate ultrahigh Q silica-based photonic devices for next generation photonic applications.

Impact of stimulated Raman scattering on dark soliton generation in a silica microresonator

Generating a coherent optical frequency comb at an arbitrary wavelength is important for fields such as precision spectroscopy and optical communications. Dark solitons which are coherent states of optical frequency combs in normal dispersion microresonators can extend the operating wavelength range of these combs. While the existence and dynamics of dark solitons has been examined extensively, requirements for the modal interaction for accessing the soliton state in the presence of a strong Raman interaction at near visible wavelengths has been less explored. Here, analysis on the parametric and Raman gain in a silica microresonator is performed, revealing that four-wave mixing parametric gain which can be created by a modal-interaction-aided additional frequency shift is able to exceed the Raman gain. The existence range of the dark soliton is analyzed as a function of pump power and detuning for given modal coupling conditions. We anticipate these results will benefit fields requiring optical frequency combs with high efficiency and selectable wavelength such as biosensing applications using silica microcavities that have a strong Raman gain in the normal dispersion regime.

Photothermal control of whispering gallery mode lasing in polymer-coated silica microcavity using high-efficiency nanoheater

Photothermal control has been widely used in the tuning of microelectronic devices due to its laser-assisted noncontact manipulation and cost-effective advantages. The Nd3+ heavily doped NaGdF4 nanocrystals (NCs) were proposed as a heat source at nanoscale, and its light-to-heat conversion efficiency was evaluated to be above 80%. Here, we obtained a whispering gallery mode lasing emission from the prepared polymer-coated silica microcavity under a 532 nm pulsed laser. The nanoheaters are combined with the microcavity to trigger the photothermal effect at the irradiation of a second 793 nm continuous wave (CW) laser, thereby realizing all-optical tuning of the resonance wavelength by changing the effective refractive index of the microcavity. By measuring the shift of lasing modes on exposure to CW laser with different power intensities, the maximum tuning range and sensitivity of 2 nm and 1.96 nm/(W·mm−2) were achieved as the power intensity increased from 0 to 1.02 W/mm2. Our work provides a method for developing a new generation of all-optical tunable lasers based on nanoscale photothermal effect.

Chaos-assisted two-octave-spanning microcombs

Since its invention, optical frequency comb has revolutionized a broad range of subjects from metrology to spectroscopy. The recent development of microresonator-based frequency combs (microcombs) provides a unique pathway to create frequency comb systems on a chip. Indeed, microcomb-based spectroscopy, ranging, optical synthesizer, telecommunications and astronomical calibrations have been reported recently. Critical to many of the integrated comb systems is the broad coverage of comb spectra. Here, microcombs of more than two-octave span (450 nm to 2,008 nm) is demonstrated through χ(2) and χ(3) nonlinearities in a deformed silica microcavity. The deformation lifts the circular symmetry and creates chaotic tunneling channels that enable broadband collection of intracavity emission with a single waveguide. Our demonstration introduces a new degree of freedom, cavity deformation, to the microcomb studies, and our microcomb spectral range is useful for applications in optical clock, astronomical calibration and biological imaging.

Microcavity Nonlinear Optics with an Organically Functionalized Surface.

We report enhanced optical nonlinear effects at the surface of an ultrahigh-Q silica microcavity functionalized by a thin layer of organic molecules. The maximal conversion efficiency of third harmonic generation reaches ∼1680%/W^{2} and an absolute efficiency of 0.0144% at pump power of ∼2.90 mW, which is approximately 4 orders of magnitude higher than that in a reported silica microcavity. Further analysis clarifies the elusive dependence of the third harmonic signal on the pump power in previous literature. Molecule-functionalized microcavities may find promising applications in high-efficiency broadband optical frequency conversion and offer potential in sensitive surface analysis.

Absorption and gain saturable nonlinearities in erbium-doped optical microcavities

Nonlinear non-Hermitian systems with saturable nonlinearities give rise to versatile phenomena that have no counterparts in linear systems. Here, we theoretically and experimentally demonstrate saturable nonlinearity (saturable absorption and gain) of erbium ions doped in silica microcavities at different concentrations. Our results show that when the doped concentration is low, the single isolated model agrees well with the experimental results, and also can be used as a basic description of highly doped samples to analyze the saturable nonlinearity. Yet, for highly doped microcavities we find that the clustering effect shall be also taken into account in order to match experimental data and to achieve a reasonable agreement. To confirm our finding, we experimentally characterize the saturable nonlinearity for ion concentrations varying from $\phantom{\rule{4pt}{0ex}}1.0\phantom{\rule{4pt}{0ex}}$ to $5.0\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Our experiment shows excellent consistency with the theory, which in turn suggests other potential applications of the theory based on saturable nonlinearity, including optical bistability and nonlinear parity-time symmetry.

Low threshold anti-Stokes Raman laser on-chip

Raman lasers based on integrated silica whispering gallery mode resonant cavities have enabled numerous applications from telecommunications to biodetection. To overcome the intrinsically low Raman gain value of silica, these devices leverage their ultra-high quality factors (Q), allowing sub-mW stimulated Raman scattering (SRS) lasing thresholds to be achieved. A closely related nonlinear behavior to SRS is stimulated anti-Stokes Raman scattering (SARS). This nonlinear optical process combines the pump photon with the SRS photon to generate an upconverted photon. Therefore, in order to achieve SARS, the efficiency of the SRS process must be high. As a result, achieving SARS in on-chip resonant cavities has been challenging due to the low lasing efficiencies of these devices. In the present work, metal-doped ultra-high Q (Q>107) silica microcavity arrays are fabricated on-chip. The metal-dopant plays multiple roles in improving the device performance. It increases the Raman gain of the cavity material, and it decreases the optical mode area, thus increasing the circulating intensity. As a result, these devices have SRS lasing efficiencies that are over 10x larger than conventional silica microcavities while maintaining low lasing thresholds. This combination enables SARS to be generated with sub-mW input powers and significantly improved anti-Stokes Raman lasing efficiency.

Tuning on Optical Resonances of Microcavities Based on Thermal Dissipation Control

Optical resonances in a microcavity can be optically or thermally manipulated. Here, the tunings on optical resonances in silica microcavities with various dimensions are achieved by controlling the thermal dissipation with a thermal conductor. Finite-element thermal analyses show that the temperature in a mode volume of a microcavity can be definitely tuned, and the shifts of resonances near $1.55~\mu \text{m}$ are verified experimentally in a ~160- $\mu \text{m}$ -diameter cavity. In addition, the slight resonance shifts in a ~450- $\mu \text{m}$ -diameter cavity are examined indirectly from the output of single-mode Brillouin lasing. Compared to that of optical coupling, the thermal interaction space is much larger and fulfills a need to manipulate the high-quality resonances in a robust way.

Silica-microsphere-cavity-based microwave photonic notch filter with ultra-narrow bandwidth and high peak rejection.

We propose and experimentally demonstrate a microwave photonic (MWP) notch filter based on a silica microsphere cavity. By using a high-Q-factor (∼1e7) cavity with a diameter of 132um, the filter bandwidth can be easily decreased to 15MHz in terms of simple fabrication and flexible coupling. Then we use the advanced modulation technique based on a dual parallel Mach-Zehnder modulator to further improve peak rejection (PR). The experimental results show that the MWP notch filter with its PR beyond 55dB and frequency tunability range over 8GHz has been achieved in combination with double-sideband modulation. To the best of our knowledge, this is a record for PR and bandwidth considered simultaneously for an MWP filter based on silica microcavities. Thus, the proposed MWP filter will be useful in the fields of microwave photonic signal processing, radar systems, etc.

Symmetry-breaking-induced nonlinear optics at a microcavity surface

Second-order nonlinear optical processes lie at the heart of many applications in both classical and quantum regimes1,2,3. Inversion symmetry, however, rules out the second-order nonlinear electric-dipole response in materials widely adopted in integrated photonics (for example, SiO_2, Si and Si_3N_4). Here, we report nonlinear optics induced by symmetry breaking at the surface of an ultrahigh-Q silica microcavity under a sub-milliwatt continuous-wave pump. By dynamically coordinating the double-resonance phase matching, a second harmonic is achieved with an unprecedented conversion efficiency of 0.049% W^(−1), 14 orders of magnitude higher than that of the non-enhancement case. In addition, the nonlinear effect from the intrinsic symmetry breaking at the surface can be identified unambiguously, with guided control of the pump polarization and the recognition of the second-harmonic mode distribution. This work not only extends the emission frequency range of silica photonic devices, but also lays the groundwork for applications in ultra-sensitive surface analysis.

All-Optical Tunable Microlaser Based on an Ultrahigh-Q Erbium-Doped Hybrid Microbottle Cavity

An all-optical tunable microlaser based on the ultrahigh-quality (Q)-factor erbium-doped hybrid microbottle cavity is proposed and experimentally demonstrated for the first time. All-optical wavelength tunability of the silica microcavity laser is a very attractive feature and has been rarely reported. By using an improved doping method, the erbium-doped silica microbottle cavity with a Q factor of 5.2 × 107 in the 1550 nm band is obtained, which is higher than the previous work based on the conventional sol–gel method. Through nonresonant pump in the 980 nm band, a lasing threshold of 1.65 mW is achieved, which is lower than all those realized through the same pump method. Besides, iron oxide nanoparticles are coated on the tapered area by doping them in the ultraviolet curing adhesive in order to precisely control the coating area, which enables the hybrid microcavity to maintain the ultrahigh Q factor and possess large tunability. By feeding the control light through the axial direction of the microbot...

Graphene-supported high-efficient modulation based on electromagnetically induced transparency in silica microcavity

The combination of graphene and high quality (Q) factor microcavity provides a promising way for realizing high-efficient modulation. In this paper, an electro-optic tuning of a graphene–silica microdisk is investigated. Two graphene flakes, separated by Al2O3 isolation layer, are embedded in the silica microdisk to significantly enhance the light-matter interaction for the achievement of high-efficient modulation. Maximal resonant wavelength shift of 9.5 nm is presented as the bias voltage of about 189.5 V is applied on the graphene flakes, which is beneficial for realizing the optical modulation with the extinction ratio of about 16 dB. In addition, two partially reflecting elements are embedded in the waveguide which is side-coupled with the microcavity for realizing the electromagnetically induced transparency (EIT), subsequently achieving the high-efficient modulation with the extinction ratio of about 33.3 dB. Meanwhile, it is found that the 3 dB modulation bandwidth, maximum of as high as 58.1 GHz, gradually decreases as the thickness of Al2O3 isolation layer is reduced. As the potential applications, this improved study of graphene–silica microdisk opens up great potential for realizing high-efficient electro-optic devices such as modulator, optical switch and ultra-short pulsed laser.

Bridging ultrahigh-Q devices and photonic circuits

Optical microcavities are essential in numerous technologies and scientific disciplines. However, their application in many areas relies exclusively upon discrete microcavities in order to satisfy challenging combinations of ultra-low-loss performance (high cavity-Q-factor) and cavity design requirements. Indeed, finding a microfabrication bridge connecting ultra-high-Q device functions with micro and nanophotonic circuits has been a long-term priority of the microcavity field. Here, an integrated ridge resonator having a record Q factor over 200 million is presented. Its ultra-low-loss and flexible cavity design brings performance that has been the exclusive domain of discrete silica and crytalline microcavity devices to integrated systems. Two distinctly different devices are demonstrated: soliton sources with electronic repetition rates and high-coherence Brillouin lasers. This multi-device capability and performance from a single integrated cavity platform represents a critical advance for future nanophotonic circuits and systems.

Transition between Kerr comb and stimulated Raman comb in a silica whispering gallery mode microcavity

We theoretically and experimentally investigated the transition between modulation instability and Raman gain in a small silica microcavity with a large free-spectral range (FSR), which reveals that we can selectively switch from a four-wave mixing dominant state to a stimulated Raman scattering dominant state. Both the theoretical analysis and the experiment show that a Raman-dominant region is present between transitions of Kerr combs with different free-spectral range spacings. We can obtain a stable Kerr comb and a stable Raman state selectively by changing the driving power, coupling between the cavity and the waveguide, and laser detuning. Such a controllable transition is achieved thanks to the presence of gain competition between modulation instability and Raman gain in silica whispering gallery mode microcavities.

All-optical control of ultrahigh-Q silica microcavities with iron oxide nanoparticles.

We propose and experimentally demonstrate, to the best of our knowledge, the first all-optical control scheme of ultrahigh-quality (Q)-factor silica microcavities, which can maintain their Q factors over 108 during the tuning process. For silica microcavities, the resonance tunability is very important and is also challenging for many applications. However, almost all previous works on resonance tuning deteriorate the Q factors of silica microcavities at different levels, and evidently these schemes are not suitable for applications in which ultrahigh Q factors are required. In this work, based on the proposed silica microbottle cavity and iron oxide nanoparticles, we realize all-optical control of the silica microcavity and maintain its Q factor of around 1.2×108 during the tuning process. A tuning range of 85.9GHz (0.68nm) and a tuning sensitivity of 13.6GHz/mW are obtained, and it is possible to realize full tunability by bridging the azimuthal free spectral range using six adjacent q-series modes. Moreover, all-optical control of the reflection spectrum is also carried out. This work will broaden the applications of ultrahigh-Q silica microcavities in nonlinear optics, microwave photonics, cavity optomechanics, and cavity quantum electrodynamics.

Transverse mode interaction via stimulated Raman scattering comb in a silica microcavity.

Comb generation in different mode families via a stimulated Raman scattering (SRS) process is studied using a silica toroid microcavity. The broad gain bandwidth of SRS in silica allows us to excite longitudinal modes at long wavelengths belonging to mode families that are either the same as or different from the pump mode. We found through experiment and numerical analysis, that an SRS comb in a different mode family with a high quality factor (Q) is excited when we pump in a low-Q mode. No transverse mode interaction occurs when we excite in a high-Q mode resulting the generation of a single comb family. We studied the condition of the transverse mode interaction while varying the mode overlap and Q of the Raman mode. Our experimental results are in good agreement with the analysis and this enables us to control the generation of one- and two-mode combs.

High Efficiency Raman Lasers Based on Zr-Doped Silica Hybrid Microcavities

Raman lasers form a particularly unique and versatile class of optically pumped laser. Because they rely on stimulated Raman scattering (SRS), which is governed by the vibrational frequency of the gain material and not electronic transitions, the emission wavelength is determined by the pump wavelength, enabling broad tunability. Ultrahigh quality factor (Q) silica optical resonators are an ideal platform for Raman lasers because the long photon lifetime results in large circulating optical intensities. Previous work has demonstrated extremely low thresholds using these devices. However, only moderate Raman lasing efficiencies have been achieved in silica devices. In the present work, we demonstrate that a zirconium (Zr)-doped silica sol–gel coating can improve the performance of a silica microcavity Raman laser. Several concentrations of Zr-doped sol–gel are synthesized. The intrinsic Raman gain of the Zr-doped silica is measured using Raman spectroscopy, and the values show a clear dependence on Zr dopa...

Theory and measurement of the soliton self-frequency shift and efficiency in optical microcavities.

Dissipative Kerr cavity solitons experience a so-called self-frequency shift (SFS) as a result of Raman interactions. The frequency shift has been observed in several microcavity systems. The Raman process has also been shown numerically to influence the soliton pumping efficiency. Here, a perturbed Lagrangian approach is used to derive simple analytical expressions for the SFS and the soliton efficiency. The predicted dependences of these quantities on soliton pulse width are compared with measurements in a high-Q silica microcavity. The Raman time constant in silica is also inferred. Analytical expressions for the Raman SFS and soliton efficiency greatly simplify the prediction of soliton behavior over a wide range of microcavity platforms.

Thermal Effects on Kerr Comb Generation in a CaF2Whispering-Gallery Mode Microcavity

We numerically investigate the dynamics of Kerr comb generation in a calcium fluoride whispering gallery mode (WGM) microcavity taking thermal effects into account. Soliton pulse generation is investigated in the presence of thermooptic (TO) and thermal expansion (TE) effects and particularly when the TO effect is negative. We found that, with optimized cavity parameters, self-induced cavity resonance shifts resulting from a negative TO effect allow the system to generate a soliton pulse with no wavelength scanning or dynamic power optimization, which are usually needed to achieve a soliton pulse with silica and other WGM microcavity systems. A negative TO effect automatically tunes the cavity resonance so that the cavity will achieve a soliton state. The competition between the TO and TE effects is also investigated in detail.